Mechanistic analysis of solvent extraction of heavy metals in membrane contactors

The extraction of Zn2+ and Cu2+ in a hollow fiber membrane module with a kerosene solution of di(2-ethylhexyl)phosphoric acid (D2EHPA) was studied. Experiments were performed at different aqueous metal concentrations (0.78–15.7 mol/m3), pH2–6, and organic D2EHPA concentrations (25–100 mol/m3). A mass transfer model was presented that considers aqueous layer diffusion, membrane diffusion, and organic layer diffusion. The calculated time profiles of aqueous metal concentrations agreed reasonably with the measured data (standard deviation, 9%) based on a good knowledge of the chemistry of extraction reactions and the transport properties of the relevant geometry. By comparing the relative resistance of each diffusion step, the extraction of Zn2+ was predominantly governed by aqueous layer diffusion; however, the diffusion in aqueous layer, membrane, and organic layer all played a certain role for the extraction of Cu2+. The changes of rate-controlling step(s) with time during the experiments were also evaluated.